measuring the serum Ig concentration and bacteria infection

I would like to detect if my KO mice developed immunodeficiency. One assay is comparing the serum Ig concentration of both WT and KO mice. Normally the references suggest using the 7 to 10 weeks mice. Why have to be 7 to 10 weeks old? Are there problems if they are elder or younger? And do I have to use the WT and KO mice with the exactly same age. Would it be a problem if one is one week younger? I used the ELISA to measuring the serum Ig concentration. Would my OD value be too high if > 2.0?

The other thing is, did any of you ever use bacteria, LPS or virus to infect the mice to see the immunodeficiency? What is the difference between these three? And where can I get them?

The age of mice will matter believe it or not. The age range suggested is appropriate for optimal immune induction. Does this mean younger or older mice will not develop immunity? No. However, age does matter. If you do not intend to look into this experimentally, then I would suggest you stick with the recommendation.